The present invention relates to a mechanism for increasing the versatility of stringed musical instruments, such as guitars and mandolins, equipped for electronic amplification by shifting the bridge support to alter the coupling of the instrument strings with the instrument soundboard to produce a hollow body sound, a solid body sound, or a combination of the two.
When a large amount of energy is transferred between the vibrating strings and the body of a stringed musical instrument, the instrument will produce a significant volume of sound without the use of amplification. A low amount of energy transfer will reduce the unamplified volume, but has the benefit of allowing the strings to vibrate for a significantly longer time after being plucked, due to the decrease in energy dissipated by vibration of the body of the instrument. This increased vibration time is commonly called sustain by musicians and is very desirable for some styles of music. Another concern of the musician relating to the vibrational traits of a stringed instrument used with an amplification system is the chance of acoustic feedback being generated during a performance. Acoustic feedback is produced when the sound produced by the instrument is amplified and the force of the sound from the amplifier is strong enough and in the right phase to cause the strings to vibrate more, making the sound louder. Thus, an instrument that vibrates less easily is less likely to fall victim to feedback. Accordingly, a rough rule of thumb is that resistance to feedback and sustain are inversely proportional to unamplified volume capability.
These different possible proportions of volume to sustain/feedback resistance are due to the motion of the strings being coupled to the soundboard of the instrument. As the soundboard flexes, the kinetic energy of the string is converted into sound and, to some extent, heat resulting from friction. As a general rule, the more vibration that is passed from the instrument strings to the soundboard, the greater the amount of motion in the soundboard. This results in more sound being produced, but it results in the kinetic energy of the string being used up more quickly.
Since the two factors involved have an inversely proportional relation, two theoretical extremes are possible. First, all of the kinetic energy of the strings could be converted instantaneously to sound. This would result in a very short but loud sound, after which the strings would be perfectly still. The other extreme would be for the energy of the strings to be dissipated only in the strings with no vibration at all in the soundboard. This would result in quiet sound that lasted for a longer time. It would be quiet because the string has far less surface area to transfer motion into sound and it would last longer because the energy would be converted at a slower rate.
In addition to the relationship between the sound produced by an instrument string and the duration of string vibration, one additional factor must be considered. A given area of vibrational surface will produce different amounts of sound depending on how easily it vibrates. In general, a rigid surface will not vibrate, but will instead reflect the vibration back into the strings while a springy surface will begin to vibrate with the string, thereby producing sound. When surface characteristics are taken into account, it can be seen that the performance of an instrument soundboard is dependent on both its area and vibrational characteristics. A solid body, however, is far less sensitive to changes in size so long as it maintains its rigidity. This is because the amount of vibration in a solid body per unit of area is so small that changing the area has little effect on the system. The size of a solid body tends to be significant only as it impacts rigidity and structural strength.
All three characteristics of instrument performance related to ease of vibration (unamplified volume on one side of the spectrum, increased sustain and feedback resistance on the other) are desirable qualities in an instrument. Due to the fact that they are somewhat inversely related, however, most instruments are designed to emphasize either unamplified volume or the combination of sustain and feedback resistance. The present invention seeks to provide a means for a single instrument t be switched between distinct modes of operation with each mode having a different combination of volume, tone and sustain characteristics.